Icy comet yields hot minerals

Tiny speck of comet dust, less than 500 micrometres long, and its track embedded in low-density glass (Image: University of California/NASA)

Scientists studying comet samples returned by the Stardust spacecraft have uncovered a cosmic conundrum. How did minerals that form in extremely hot temperatures end up inside an icy comet?

"When they formed, they were either red-hot or white-hot and we found them in the Siberia of the solar system," says Stardust lead scientist Professor Donald Brownlee, with the University of Washington.

The Stardust spacecraft was launched in February 1999 and flew by Comet Wild 2 (pronounced 'vilt 2') five years later.

The comet formed in the frigid Kuiper Belt region located beyond Neptune's orbit and was a relatively new traveller to the inner solar system when it encountered the NASA probe.

Samples from the comet were collected by the spacecraft in trays of exposed gel and returned to Earth in January.

The discovery of exotic, high-temperature minerals rich in calcium, aluminum and titanium complicate generally accepted theories about how comets form.

Far from simple icy bodies with clouds of gas and dust, comets, or at least Comet Wild 2, now appear to be a blend of materials formed at all temperature ranges.

Forging metalsBrownlee and other Stardust researchers expect to learn whether the high-temperature metals were forged near the center of the nebular cloud that eventually became our solar system or whether they come from another star.

"If it was formed in our solar system, then it had to be transported from the hottest regions to the coolest," says Brownlee.

The samples also show a rich supply of olivine, an iron-magnesium blend of minerals, which on Earth can be found in peridot gemstones and in the green sand on some Hawaiian beaches.

Though it is one of the most common minerals in the universe, scientists did not expect to find it in a comet.

This particle, a type of olivine in the centre of the image, was brought to Earth in the Stardust sample-return capsule. It's embedded in low-density glass (Image: University of Washington)

Olivine forms in temperatures of about 1100°C, which means the mineral would have had to form closer to the Sun than Mercury and then somehow be flung out more than 45 times the distance of the Sun to Earth.

Researchers plan extensive studies to determine the chemical histories of the sample particles, which will eventually reveal whether the grains came from our solar system or another star.

Between 150 and 200 samples from the comet are currently circulating among scientists and research labs across the world.

Next month, work begins on another set of samples collected during the mission: interstellar dust grains.

Unlike the comet samples, the bits of interstellar dust picked up during Stardust's travels are tiny.

Scientists plan an internet-based detection program of volunteers who will use their home computers to scrutinise images and try to identify any particles. The project, called Stardust@Home, begins in April.